The refractive indices of transparent media, such as, for instance, glasses, are at low light intensities generally independent of intensity, but become dependent thereon at higher intensities. More generally, the refractive index of a material typically is a function of electric field. For purposes of this application, the relevant electric field is the electric field of the electromagnetic signal transmitted through a transparent medium, and the term "electric field" is used herein in this sense. This field-dependence of the refractive index (or, more generally, of a generalized susceptibility) results in a plethora of physical phenomena, the study of which constitutes the field of nonlinear optics. See, for instance, Quantum Electronics: A Treatise, Vol. 1, H. Rabin and C. L. Tang, Editors, Academic Press, New York (1975).
Lightwave communication systems almost universally employ lightguides as transmission media. These guides, also referred to as "optical fiber", typically comprise a core region having a maximum (field-independent) refractive index n.sub.o, and a cladding having refractive index n.sub.1, with n.sub.1 &lt;n.sub.o, at least for a wavelength regime containing the operating wavelength of the system, in order to achieve guiding by means of total internal reflection. Lightguides of course can be designed for operation with other than visible electromagnetic radiation, and I will use the terms "optical" and "light" in this broader sense. In particular, I intend the terms to include the near infrared region of the spectrum.
The possible rate of information transmission through an optical fiber is generally expressed in terms of bandwidth, or of a maximum bit rate per second. This rate is limited, inter alia, by dispersion in the fiber, since, as light pulses travel through a fiber, typically each of the pulses broadens and eventually overlaps with its neighbors, resulting in an increase in the number of errors in the transmitted information. Three mechanisms are typically responsible for this pulse broading in fibers: material dispersion, waveguide dispersion, and modal dispersion. Of these, waveguide dispersion typically can be neglected because, inter alia, modes subject to substantial waveguide dispersion generally are selectively attenuated in multimode fiberguides. Material dispersion will typically also be small for conditions of interest in this application. For these reasons, and since this application is concerned with a method for reducing mode dispersion, the discussion of dispersion will be from here on substantially limited to mode dispersion.
As is well known, Maxwell's equations, when solved for the case of a fiber lightguide, have guided wave solutions only for a discrete set of waves. These allowed solutions, referred to as guided modes, can be labeled by two mode numbers, of which one describes, in the geometrical optics picture, the angle the ray of the wave makes with the axis of the fiber. Employing this geometrical optics picture, it is easy to see that modes differing in mode number will generally have different path lengths in the fiber, leading to pulse spreading due to mode dispersion. This spreading can be reduced by properly shaping the refractive index profile of the core region of lightguides. See for instance, Optical Fiber Communications, S. E. Miller and A. G. Chynoweth, Editors, Academic Press, New York (1979), especially Chapter 3, pages 37-100. However, no profile is known that will result even in theory in zero mode dispersion in fibers carrying more than two modes. Furthermore, practical limitations in fiber manufacture make exact reproduction of a theoretical profile difficult to achieve, resulting typically in fibers having actual bandwidth less than the theoretically achievable maximum. Since on the other hand wavelength regimes exist in which material dispersion is small and in which modal dispersion is the dominant dispersion mechanism, a method for reducing modal dispersion beyond what is achievable by means of fiber design is clearly of considerable practical interest. Such a method can lead to improved multimode optical fiber data transmission systems.